The new judge presiding over the James "Whitey" Bulger case has granted prosecutors' motion to bar the reputed Boston mobster from arguing immunity at trial.

Judge Denise Casper upheld a decision by the case's previous judge, who was removed because of a potential conflict of interest.

Judge Casper ruled "any alleged immunity agreement ... is a bar to prosecution to be determined by the court and not a defence to the crimes charged".

Bulger's lawyers argued they should be able to present a trial defence claiming that a now-dead law enforcement official gave Bulger immunity from prosecution.

US District Judge Richard Stearns was removed from the case in March after defence lawyers cited a possible conflict of interest due his role as a federal prosecutor during the 1980s - the same time Bulger was working as an FBI Mafia informant while allegedly committing crimes - including murder.

Bulger, 83, is scheduled to go on trial next month for his connection to 19 slayings. He has pleaded not guilty.

The reputed gangster fled Boston in 1994 and was on the FBI's most wanted list until he was captured in Southern California in 2011.